Effect of solution treatment on the microstructure,phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials.In this work,a ternary(NiTi)91Nb9(at.%)shape memory alloy was produced by laser powder bed fusion(L-PBF)using pre-alloyed NiTi and elemental Nb powders.The effect of solution treatment on the microstructure,phase transformation behavior and mechanical/functional performances was investigated.The in-situ alloyed(NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix.Upon high-temperature(1273 K)solution treatment,Nb-rich precipitates were precipitated from the supersaturated matrix.The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment,leading to a morphological transition from mesh-like into rod-like and sphere-like.Coarsening of theβ-Nb phases occurred with increasing holding time.The martensite transformation temperature increases after solution treatment,mainly attributed to:(i)reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi,and(ii)the Ti expulsion from theβ-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix,which resulted from the microstructure changes from non-equilibrium to equilibrium state.The thermal hysteresis of the solutionized alloys is around 145 K after 20%pre-deformation,which is comparable to the conventional NiTiNb alloys.A short-term solution treatment(i.e.at 1273 K for 30 min)enhances the ductility and strength of the as-printed specimen,with the increase of fracture stress from(613±19)MPa to(781±20)MPa and the increase of fracture strain from(7.6±0.1)%to(9.5±0.4)%.Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates>90%.This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

Laser powder bed fusion additive manufacturing of NiTi shape memory alloys: a review

NiTi alloys have drawn significant attentions in biomedical and aerospace fields due to their unique shape memory effect(SME),superelasticity(SE),damping characteristics,high corrosion resistance,and good biocompatibility.Because of the unsatisfying processabilities and manufacturing requirements of complex NiTi components,additive manufacturing technology,especially laser powder bed fusion(LPBF),is appropriate for fabricating NiTi products.This paper comprehensively summarizes recent research on the NiTi alloys fabricated by LPBF,including printability,microstructural characteristics,phase transformation behaviors,lattice structures,and applications.Process parameters and microstructural features mainly influence the printability of LPBF-processed NiTi alloys.The phase transformation behaviors between austenite and martensite phases,phase transformation temperatures,and an overview of the influencing factors are summarized in this paper.This paper provides a comprehensive review of the mechanical properties with unique strain-stress responses,which comprise tensile mechanical properties,thermomechanical properties(e.g.critical stress to induce martensitic transformation,thermo-recoverable strain,and SE strain),damping properties and hardness.Moreover,several common structures(e.g.a negative Poisson’s ratio structure and a diamond-like structure)are considered,and the corresponding studies are summarized.It illustrates the various fields of application,including biological scaffolds,shock absorbers,and driving devices.In the end,the paper concludes with the main achievements from the recent studies and puts forward the limitations and development tendencies in the future.

Effect of Co substitution on magnetic properties of Ni-Mn-Sn magnetic shape memory alloys

The effect of Co substitution on magnetic properties of Ni-Mn-Sn shape memory alloy was revealed by first-principles calculations. Large magnetization difference in Ni-Mn-Sn alloy obtained by addition of Co arises from enhancement of magnetization of austenite due to change of Mn-Mn interaction from anti-ferromagnetism to ferromagnetism. Total energy difference between paramagnetic and ferromagnetic austenite plays an important role in magnetic transition of Ni-Co-Mn-Sn. The altered Mn 3d states due to Co substitution give rise to difference in magnetic properties.

Effects of second phases on mechanical properties and martensitic transformations of ECAPed TiNi and Ti-Mo based shape memory alloys

TiNi and Ti-based shape memory alloys were processed by equal channel angular pressing （ECAP） at 673-773 K along Bc route to obtain ultrafine grains for increasing the strength of parent phase and improving the functional properties. The effects of both thermodynamically stable and metastable second phases on the mechanical properties and martensitic transformations of these alloys were investigated. It is found that thermodynamically stable Ti2Ni phase has no effect on martensitic transformation and superelasticity of Ti-rich TiNi alloy, thermodynamically stable α phase is harmful for ductility of Ti-Mo-Nb-V-Al alloy, but metastable Ti3Ni4 phase is effective for R phase transformation, martensitic transformation and superelasticity of Ni-rich TiNi alloy. The mechanisms of the second phases on the martensitic transformations and mechanical properties were discussed.

Review on non-conventional machining of shape memory alloys

Shape memory alloys （SMAs） are the developing advanced materials due to their versatile specific properties such as pseudoelasticity, shape memory effect （SME）, biocompatibility, high specific strength, high corrosion resistance, high wear resistance and high anti-fatigue property. Therefore, the SMAs are used in many applications such as aerospace, medical and automobile. However, the conventional machining of SMAs causes serious tool wear, time consuming and less dimensional deformity due to severe strain hardening and pseudoelasticity. These materials can be machined using non-conventional methods such as laser machining, water jet machining （WJM） and electrochemical machining （ECM）, but these processes are limited to complexity and mechanical properties of the component. Electrical discharge machining （EDM） and wire EDM （WEDM） show high capability to machine SMAs of complex shapes with precise dimensions. The aim of this work is to present the consolidated references on the machining of SMAs using EDM and WEDM and subsequently identify the research gaps. In support to these research gaps, this work has also evolved the future research directions.

Influences of 2.5wt%Mn addition on the microstructure and mechanical properties of Cu-Al-Ni shape memory alloys

The influences of 2.5wt%Mn addition on the microstructure and mechanical properties of the Cu-11.9wt%Al-3.8wt%Ni shape memory alloy（SMA） were studied by means of scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,and differential scanning calorimeter（DSC）.The experimental results show that Mn addition influences considerably the austenite-martensite transformation temperatures and the kind of martensite in the Cu-Al-Ni alloy.The martensitic transformation changes from a mixedβ1→β＇1＋γ＇1 transformation to a singleβ1→β＇1 martensite transformation together with a decrease in transformation temperatures.In addition,the observations reveal that the grain size of the Cu-Al-Ni alloy can be controlled with the addition of 2.5wt%Mn and thus its mechanical properties can be enhanced.The Cu-Al-Ni-Mn alloy exhibits better mechanical properties with the high ultimate compression strength and ductility of 952 MPa and 15%,respectively.These improvements are attributed to a decrease in grain size.However,the hardness decreases from Hv 230 to Hv 140 with the Mn addition.

EFFECT OF MICROSTRUCTURE ON THE HARDENING AND SOFTENING BEHAVIORS OF POLYCRYSTALLINE SHAPE MEMORY ALLOYS PART Ⅰ:MICROMECHANICS CONSTITUTIVE MODELING

The effects of microstructure and its evolution on the macroscopic superelastic stress-strain response of polycrystalline Shape Memory Alloy(SMA)are studied by a microstructure-based constitutive model developed in this paper.The model is established on the following basis:(1)the transformation conditions of the unconstrained single crystal SMA microdomain(to be distinguished from the bulk single crystal),which serve as the local criterion for the derivation of overall transfor- mation yield conditions of the polycrystal;(2)the micro-to macro-transition scheme by which the connection between the polycrystal aggregates and the single crystal microdomain is established and the macroscopic transformation conditions of the polycrystal SMA are derived;(3)the quantitative incorporation of three microstruc- ture factors(i.e.,nucleation,growth and orientation distribution of martensite)into the modeling.These microstructural factors are intrinsic of specific polycrystal SMA systems and the role of each factor in the macroscopic constitutive response is quan- titatively modeled.It is demonstrated that the interplay of these factors will result in different macroscopic transformation kinematics and kinetics which are responsible for the observed macroscopic stress-strain hardening or softening response,the latter will lead to the localization and propagation of transformation bands in TiNi SMA.

Role of Ag addition on microstructure,mechanical properties,corrosion behavior and biocompatibility of porous Ti-30 at%Ta shape memory alloys

In the present study,the thermal,mechanical,and biological properties of xAg/Ti-30Ta(x=0,0.41,0.82 and 2.48 at%)shape memory alloys(SMAs)were investigated.The study was conducted using optical and scanning electron microscopy(SEM),X-ray diffractometry(XRD),compression test,and shape memory testing.The xAg/Ti-Ta was made using a powder metallurgy technique and microwave-sintering process.The results revealed that the addition of Ag has a significant effect on the pore size and shape,whereas the smallest pore size of 11μm was found with the addition of 0.41 at%along with a relative density of 72%.The fracture stress and strain increased with the addition of Ag,reaching the minimum values around 0.41 at%Ag.Therefore,this composition showed the maximum stress and strain at fracture region.Moreover,0.82 Ag/Ti-Ta shows more excellent corrosion resistance and biocompatibility than other percentages,obtaining almost the same behaviour of the pure Ti and Ti-6Al-4V alloys,which can be recommended for their promising and potential response for biomaterial applications.

Martensitic transformation in Cu-doped NiMnGa magnetic shape memory alloys

This paper studies the martensitic transformation in the Cu-doped NiMnGa alloys. The orthorhombic martensite transforms to L21 cubic austenite by Cu substituting for Ni in the Nis0-xCuxMn31Ga19 （x=2 10） alloys, the martensitic transformation temperature decreases significantly with the rate of 40 K per Cu atom addition. The variation of the Fermi sphere radius （kF） is applied to evaluate the change of the martensitic transformation temperature. The increase of kF leads to the increase of the martensitic transformation temperature.

Effects of Ta Addition on NiTi Shape Memory Alloys

The effect of Ta addition on the martensitic transformation characteristics and the X-ray visibility on NiTi shape memory alloy have been studied in (Ni51Ti49)1-xTax system. It was found that the transformation temperatures of the Ni51Ti49 binary alloy increased drastically by an addition of 0～4 at. pet Ta, but only slightly when the concentration exceeded 4 at. pct; the addition of Ta greatly decreases the sensitivity of the martensitic transformations to the variation in the Ni-Ti ratio. The addition of Ta to the NiTi binary alloy can improve its X-ray visibility.

EFFECT OF MICROSTRUCTURE ON THE HARDENING AND SOFTENING BEHAVIORS OF POLYCRYSTALLINE SHAPE MEMORY ALLOYS PART Ⅱ:NUMERICAL SIMULATION UNDER AXISYMMETRICAL LOADING

Based on the microstructure-based constitutive model established in Part Ⅰ,a detailed numerical investigation on the role of each microstructure pa- rameter in the kinematical and kinetic evolution of polycrystalline SMA under ax- isymmetrical tension loading is performed.Some macroscopic constitutive features of stress-induced martensite transformation are discussed.

Martensitic Transformation in Fe-Mn-Si Based Shape Memory Alloys

The critical driving force for martensitic transformation fcc ( gamma ) yields hcp ( epsilon ) in ternary Fe-Mn-Si alloys increases with the content of Mn and decreases with that of Si. Thermodynamical prediction of M//s in ternary Fe-Mn-Si alloys was established. The fcc ( gamma ) yields hcp ( epsilon ) martensitic transformation in Fe-Mn-Si is a semi-thermoelastic and the nucleation process does not strongly depend on soft mode. Nucleation occurs directly through an overlapping of stacking fault rather than pole mechanism, and it is suggested that stacking fault energy (SFE) is the main factor controlling nucleation. Based on the phenomenological theory of martensite crystallography, a shuffle on (0001)//h//c//p plane is required when d//1//1//1 does not equal d//0//0//0//2. The derived principal strain in Bain distortion is smaller, i, e., more reasonable than the values given by Christian. Alloying elements strengthening the austenite, lowering SFE of gamma phase and reducing T//N** gamma temperature may be beneficial to shape memory effect of Fe-Mn-Si based alloys. Accordingly, Fe-Mn-Si-RE and Fe-Mn-Si-Cr-N (or Fe-Mn-Si-Ni-Cr-N) are worthy to be recommended as shape memory materials with improved shape memory effect. (Edited author abstract) 48 Refs.

Kinetic grain growth, shape memory and corrosion behavior of two Cu-based shape memory alloys after thermomechanical treatment

Metallurgical and mechanical properties along with shape memory and corrosion behavior of Cu-11.8% AI-3.7% Ni-1 %Mn and Cu-11% A1 5.6% Mn shape memory alloys （SMAs） were comparatively studied. The influence of grain refinement on the properties was studied by optical microscopy （OM）, scanning electron microscopy （SEM）, differential scanning calorimetry （DSC）, potentiodynamic polarizations and bend and tensile tests. Static recrystallization and kinetic grain growth show a rapid recrystallization in the first 15 s of annealing at 800℃ followed by grain growths. The minimum grain sizes obtained after 15 s are 90 and 260 μm for Cu-A1-Ni-Mn and Cu-A1-Mn, respectively. Tensile tests show typical three-stage curves for both alloys, and it is seen that alloys exhibit high fracture stress and strain after grain refinement. Microstructural observations show zig-zag morphology of β 1martensite in the Cu-A1-Ni-Mn and coexistence of β1 and y1 in the Cu-A1-Mn, which were confirmed by differential scanning calorimetry results. The shape memory ratios （17） of the alloys before thermomechanieal treatment, and after thermomechanical annealing at 800 ℃ for different time up to 15 min followed by water quenching, were evaluated. In addition, corrosion behavior of alloys after grain refinement was analyzed by means of potentiodynamic polarization measurements. The results showed that the anodic reactions were dominated by dissolution of copper, and Cu-AI-Ni-Mn alloy exhibits a better corrosion resistance than Cu-A1-Mn alloy.

Martensite Aging Effect and Thermal Cyclic Characteristics in Ti-Pd and Ti-Pd-Ni High Temperature Shape Memory Alloys

The effect of thermal cycling and aging in martensitic state in Ti-Pd-Ni alloys were investigated by DSC and TEM observations. It is shown that the thermal cycling causes the decreases in M, and Af temperatures in Ti50Pd50-xNix (x=10, 20, 30) alloys, but no obvious thermal cycling effect was observed in Ti50Pd50Pd40Ni10 alloys and the aging effect shows a curious feature, i.e., the Af temperature does not saturate even after relatively long time aging, which is considered to be due to the occurrence of recovery recrystallization during aging.

Surface Coating of NiTi Shape Memory Alloys with Calcium Phosphates by Dip-coating or Plasma-spraying-biological Characterization Examined by in vitro Testing Methods

The influence of different surface coatings of NiTi shape memory allays was examined using in vitro testing methods. Plates of superelastic nickel-titanium shape memory allay （ NiTi ） were coated with calcium phosphates （ hydroxyapatite ） by high-temperature plasma-spraying or by dip-coating. The biocompatibility was tested in vitro by cultivation of isolated human granulocytes and whole blood cells. As substrates, pure NiTi, plasma-spray-coated NiTi and dip-coated NiTi were used. Isolated granulocytes showed an increased adhesion to both calcium phosphate-coated NiTi samples. Compared to non-coated NiTi or dip-coated NiTi, the number of dead granulocytes adherent to plasma-sprayed surfaces was significantly increased （ p 〈 0.01 ）. Whether the d/f- ferences in apoptosis of granulocytes on dip-coated vs plasma-sprayed coatings observed are due to differences in material surface morphologies has to be analyzed in further studies. Because of the cellular interactions with the coating layers, h is likely that the results obtained are not caused by the underlying NiTi but due to the coating itself.

Formation Mechanism of Curved Martensite Structures in Cu-based Shape Memory Alloys

The curved martensite structures have been observed in CuZnAI-based shape memory alloys by both transmission electron microscope and optical microscope. It was found that the curved martensite structures observed in as-solution treated, as-aged and as-trained alloys usually occurred around dislocation tangles or precipitate, at the plate boundary or grain boundary, and when the growing plates collided with each other or alternate mutually.

Martensitic stabilization and defects induced by deformation in TiNi shape memory alloys

Martensitic stabilization caused by deformation in a TiNi shape memory alloy was studied.Special attention was paid to the deformed microstructures to identify the cause of martensitic stabilization.Martensitic stabilization was demonstrated by differential scanning calorimetry for the tensioned TiNi shape memory alloy.Transmission electron microscopy revealed that antiphase boundaries were formed because of the fourfold dissociation of [110]B19＇ super lattice dislocations and were preserved after reverse transformation due to the lattice correspondence.Martensitic stabilization was attributed to dislocations induced by deformation,which reduced the ordering degree of the microstructure,spoiled the reverse path from martensite to parent phase compared with thermoelastic transformation,and imposed resistance on phase transformation through the stress field.

High temperature strain glass in Ti–Au and Ti–Pt based shape memory alloys

Strain glass is a frozen short-range strain ordered state found in shape memory alloys recently, which exhibits novel properties around the ideal glass transition temperature T_(0). However, the T_(0) of current strain glass systems is still very low, limiting their potential applications and experimental studies. In this paper, we reported two new strain glass systems with relatively high T_(0). In Ti_(50)Au_(50-x)Cr_(x) alloys, the strain glass appears at x = 25, and exhibits a T_(0) of 251 K, while in Ti_(50)Pt_(50-y)Fey alloys, the strain glass takes place at y = 30, and shows a T_(0) of 272 K. Both of them are comparable with the highest T_(0) value reported so far. Moreover, the phase diagrams of main strain glass systems in Ti-based alloys were summarized. It is found that the influence of the martensitic transformation temperature of the host alloy on the T_(0) of the strain glass is limited. This work may help to design new strain glass systems with higher T_(0) above ambient temperature.

EFFECTS OF RARE EARTH MIXTURE ON MECHANICAL PROPERTIES OF CuZnAl SHAPE MEMORY ALLOYS

With thehelpofquantitative metallograph ,tensiletest,electron probeandscan electron mi croscope,theinfluencesof mixture rareearth ( RE) on the grain size, dynamics of grain growth and mechanicalpropertiesof CuZnAlshape memoryalloys wereinvestigated . Theex perimentalresultsshowsthat REcanrefinegrainsgreatly,improvethe mechanicalpropertiesremarkably andchangethetensilefracturefrom brittletypebordered grainstoplastictypeinthecondition of maintainingshape memory properties. Moreover microstructuresrevealthatREwhich accumulates on the grain boundariescan restrain grains’growing. In addition, the mechanismsofrefininggrainsizeandimproving mechanicalpropertiesarealsodiscussed.

Modeling of Ni_4Ti_3 precipitation during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys

The phase field method was applied to study the microstructure evolution of Ni4Ti3 precipitates during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys （SAMs） with two different initial Ni-contents of 51.5% and 52.5% （mole fraction）, respectively. The simulation results show that, during stress-free aging of the NiTi alloy with a low supersaturation of Ni （i.e., Ti-51.5%Ni）, the Ni4Ti3 precipitates exhibit a heterogeneous distribution with a high number density of particles at the grain boundary, leaving most of the grain interiors free of precipitates; while for the NiTi alloy with a high supersaturation of Ni （i.e., Ti-52.5%Ni）, the Ni4Ti3 precipitates show a homogeneous distribution across the entire simulation system. The stress-assisted aging can give rise to homogeneous distribution of the precipitates, regardless of the initial Ni-content; however, the distribution of variant type within the two grains is heterogeneous.